In order to extract current from solar cells it is necessary to form metal contacts to both the n-type and p-type material of the device. One method frequently used for forming these metal contacts is metal plating. This is a an attractive method for forming metal contacts to solar cells because of its potential low cost and for forming narrow lines compared with that of screen-printed silver which is used for most commercially produced silicon solar cells. Electroless plating of nickel and then copper was successfully used by BP Solar in their manufacture of Saturn silicon solar cells.
Metal plating involves the reduction of metal ions from a solution to form a metal deposit on the solar cell. Typically, metal deposits are formed at cathodic sites on the cell (i.e., where there exists a source of electrons). This source of electrons can be provided by a reducing agent in the plating solution, in a process called electroless plating, or from electrons generated, at least in part, by the photovoltaic effect when a solar cell is exposed to light in a process called “light-induced plating” (LIP) or photoplating.
Typically, prior to metallization openings (e.g., grooves) are formed through a dielectric layer (silicon dioxide or silicon nitride) to expose regions of n-type silicon. The openings can be formed using laser-scribing or other patterning techniques, such as photolithography, and can involve either in-situ doping or subsequent furnace doping of the silicon exposed at the base of the grooves to result in a heavily-doped silicon region at the base of the grooves. This heavy doping at the base of the grooves enables ohmic contacts to be formed between deposited metal and the silicon. When metal plating, the patterned dielectric layer, provided it is of high enough quality, acts as a mask for the deposition of metal with metal, in ideal situations, only plating to the silicon regions exposed by the patterning process (e.g., at the base of the grooves).
However in commercial production, for cost, simplicity, throughput and the avoidance of high temperatures, such dielectric layers are often not of good enough quality to act as a plating mask, leading to unwanted localised areas of spurious plating in areas other than where required for the metal contacts. For example, a common problem encountered in the metal plating of silicon solar cells employing a silicon nitride dielectric masking layer, and in particular where the silicon nitride has been deposited by remote plasma enhanced chemical vapour deposition (PECVD), is the formation of spurious unwanted metal deposits over the dielectric layer, in addition to the metal deposited in the grooves. The phenomenon is often referred to as “ghost plating”. In addition to increasing the effective shading for the solar cell, “ghost plating” is also undesirable because it can result in shunts, especially when the metallization process includes a metal sintering step to further reduce the contact resistance of the metal contacts.
To some extent the “ghost plating” problem can be solved by growing a thin (10-15 nm) silicon dioxide layer on the diffused silicon before depositing the silicon nitride layer. Although, this method can be effective in eliminating “ghost plating” it has several disadvantages. First it typically requires an additional high-temperature process which is undesirable for many lower-quality silicon substrates because increased exposure to high temperatures reduce carrier lifetimes. Furthermore, it increases the overall cost of fabrication process by the increased wafer handling. Second, it limits the extent to which the hydrogen from the silicon nitride layer can be used to passivate defects in the silicon substrate. The latter point is particularly relevant to multicrystalline wafers which exhibit many defects due to the presence of grain boundaries in the wafers. Multicrystalline wafers are popular commercial substrates due to their low-cost and so this second limitation is particularly pertinent.